Clear ophthalmic articles that provide good imaging qualities, while reducing the transmission of incident light into the eye, are needed for a variety of applications, including but not limited to, sun lenses, plano lenses, fashion lenses, and vision correcting ophthalmic lenses, e.g., prescription lenses. Responsive to that need, photochromic plastic ophthalmic articles have been given considerable attention because of the weight advantage they offer, vis-à-vis, glass lenses.
Photochromism is a phenomenon involving a reversible change in color of a photochromic organic or inorganic material, e.g., a chromene or silver halide salt, or an article comprising such a material. Upon exposure to a source of activating ultraviolet or other actinic radiation, a photochromic material exhibits a change in color, e.g., it becomes darker. When the activating radiation is removed or discontinued, the photochromic material returns to its original color or colorless state. Articles that have photochromic material(s) applied to or incorporated within the article exhibit this reversible change in color and consequently a reversible change in light transmission.
Photochromic plastic ophthalmic articles have been prepared by incorporating photochromic material into the substrate of the plastic article by an imbibition process. In the imbibition process, one or more photochromic materials, e.g., organic photochromic materials, are applied to the surface of the plastic article, either as a neat photochromic dye/compound or dissolved or dispersed in a polymeric or other organic solvent carrier, and then heated to cause the photochromic dye/compound(s) to diffuse into the subsurface region of the plastic article. The subsurface region of such plastic articles are reported to have sufficient free volume to allow photochromic dye/compound(s) to transform from the colorless form into the colored form, and then revert subsequently to their original colorless form.
There are, however, certain polymer matrices that do not readily imbibe photochromic dye/compound(s). In such circumstances, the photochromic dye/compound(s) either do not penetrate sufficiently into the subsurface of the polymer, or the subsurface of the polymer is reported to have insufficient free volume, which prevents the photochromic dye/compound(s) from exhibiting significant photochromic activity. Hence, these polymer matrices cannot be used as a substrate for imbibed (or internally incorporated) photochromic materials for commercially acceptable photochromic optical applications. To allow the use of such polymer matrices as substrates for photochromic articles, it has been proposed to apply organic photochromic coatings to the surface of such polymer substrates.
Multifocal ophthalmic articles are of significant commercial interest because of the large number of individuals that require multiple optical corrections for their lenses, e.g., bifocal, trifocal and progressive lenses. It is desirable, therefore, to provide photochromic multifocal ophthalmic articles that are prepared from polymers that do not readily imbibe organic photochromic materials. Applying a photochromic coating to the surface of a preformed multifocal article has met with minimum success because of coating imperfections and optical distortions that are located near the multifocal region, e.g., the raised bifocal segment. Ophthalmic articles with such coating imperfections do not meet commercially acceptable “cosmetic standards”.